Once upon a time, Microsoft was the favorite target of malware developers. As Microsoft improved the defenses in its software, though, cybercrooks moved on to easier pickings. Adobe was a prime target for a while, but Adobe followed Microsoft's lead and made its software more secure as well.
According to data from the 2014 IBM X-Force Threat Intelligence Quarterly Report, the favorite target is now Java.

There are two things that put the bulls-eye on an application or platform. The first is distribution. Attackers may be able to develop an exploit for some obscure software that is barely used, but what would be the point? If a malware developer is going to invest the time and effort to create an exploit, he or she wants it to have the greatest possible pool of potential victims.